A large group of cell surface receptors, which mediate the actions of hormones and neurotransmitters, is coupled to the activation or inhibition of intracellular enzymes or ion channels by GTP-binding regulatory proteins (G proteins). The activated G proteins transmit information about receptor occupancy to the interior of cells. We have, for many years, studied G protein coupled receptors, particularly those for the opiates. The neuroblastoma x glioma hybrid cell line, NG108-15, is richly endowed with opiate receptors, and is a particularly good source of this protein since it expresses only one of the many opiate receptors found in the brain. In the past year we have focused on the isolation of peptide fragments of opiate receptors and the determination of their amino acid sequences. Several such sequences have been obtained, and when these are confirmed, we expect to use the information to isolate cDNA clones coding for the opiate receptor. We hope to thereby learn its complete amino acid sequence and to be able to study the regulation of its synthesis and metabolism. By the combined use of specific antibodies and synthetic peptides we had earlier characterized a highly conserved receptor domain that activates G-proteins, and showed that no intracellular domain fully shares these properties. We expect to extend these studies by preparing synthetically fused intracellular domains of receptors. Because many attempts to prepare crystals of one of several brain G protein have so far failed, we are in the process of studying the conformational properties of fragments of their alpha subunits. In conjunction with these experiments, we are carrying out studies aimed at improving the methods for predicting the three dimensional structures of proteins based upon amino acid sequence information. Such methods will be important tools for interpreting the results of cloning studies since it is evident that knowledge of amino acid sequence alone will not provide great insight into the mechanisms involved in receptor action. Some years ago, we carried out a study of opiate receptor development in the neonatal rat brain that led to the paradoxical finding that delta receptors develop primarily after birth whereas inhibition of adenylate cyclase is apparently mediated by delta receptors is fully developed at birth. We have now extended this study to embryonic mouse brain and to several types of G protein and receptor. The study allows us to conclude that the kappa opiate receptors of mouse brain, which develop early in embryogenesis, are responsible for the bulk of opiate inhibition of adenylate cyclase in the neonatal brain.